Serve /documentation URLs through default service (frontend).

[isoos]

Part of #1534

• tests and the redirect logic is copied almost verbatim
• the serving handler is referenced
• tweak added to preserve the original request after sanitization, and work on that if it is in the context

[sortie]

lgtm

Though I am not quite certain how this stops redirecting to the dartdoc instance and serves itself instead.

[isoos]

Though I am not quite certain how this stops redirecting to the dartdoc instance and serves itself instead.

Oh, true, that's the dispatch.yaml. I'm not yet sure if we should remove it (but then how to deploy it), make it empty, or override the dartdoc references to default there.

[isoos]

It should be almost-empty:
https://cloud.google.com/appengine/docs/standard/python/reference/dispatch-yaml#deleting_all_dispatch_rules

PTAL

[sortie]

lgtm